Why worry about wormer resistance

Parasitic worms are also becoming resistant to the available treatments, so a control strategy based around usage of these worming products is not sustainable. They are important and useful products but, as with antibiotics, they need to be used with care.

So why should you be worried about anthelmintic resistance (wormer resistance)?

Wormer resistance is the loss of sensitivity to a drug in a worm population, that was previously sensitive to the drug. This resistance is passed on through generations of worms, as a genetic trait, so once it is on your farm it is there to stay.

It is important to note that it is not your livestock that have developed resistance, it is the parasites themselves.

What practices encourage wormer resistance?

Frequency of worming

Every time you dose an animal for parasites you increase the risk of wormer resistance on your farm. The more frequently animals are wormed, the more easily, and quickly, worms can build up resistance. This is because we only kill susceptible worms – leaving space for those resistant worms to breed and succeed.

If wormer resistance is present on your farm, you may find you need to dose more regularly, change product or you might find products ineffective. Reducing reliance on these products and using them only when necessary is a proactive approach.

Dose and move

Dosing animals and moving them to clean pasture used to be common guidance – now we understand that it is better to leave treated animals on pasture for a short time. This is so the excreted resistant worms can breed with susceptible worms on the pasture reducing the risk of breeding only resistant worm populations.


Another practice that can encourage resistance is underdosing. This is because worms only receive a small dose of product, not enough to kill them, but enough to encourage resistance.

Ensuring animals are weighed and dosed correctly will reduce the risk of resistance; or at the very least weigh the heaviest in the group and dosing for that weight. If there is a wide range of weights, split the group and weigh the heaviest in each smaller group.

Weighbands can be used effectively in first year grazing cattle and have been shown to be more effective than estimating ‘by eye’! Just ensure the weighband is suitable for your breed.

Blanket treatment

Historically, wormers have been used extensively throughout the grazing period, sometimes preventatively and often without any diagnostic indication that they are required. Often all animals in a group are treated with the same drug at the same dose. This is known as ‘blanket treatment.’ Blanket treatment is one of the main drivers of wormer resistance.

Blanket treatment using a wormer such as an avermectin is simple. Avermectins are relatively cheap, available from trade without a veterinary prescription (except in Ireland post 2022). They are easy to administer and most of them offer a level of ‘persistency’ in the animal that means you do not have to think about worming again for a long time after administration.

However, although we may see short-term benefits to our livestock, the reality is we are likely increasing the risk of resistance and negatively impacting our effective dung fauna along the way.

How can you find out if wormer resistance is present on your farm?

Effective treatment should mean that 95% parasitic worms are killed. This can be regularly monitored using Faecal Egg Count Reduction Tests (FECRT) and should be employed EVERY time you worm your livestock. Wormer resistance is not a sudden phenomenon, it builds up slowly over time and you may not notice production losses until there is up to 50% resistant worm populations. Significant weight loss, milk yield and failure to thrive will be noted.

A FECRT can be done in the same way as a normal FWEC but should be taken 7 to 16 days after wormer use (depending on the product used). If the reduction is less than 95%, it is possible that you have wormer resistance and you must discuss this with your vet.

Before wormer resistance can be confirmed, you should rule out other possibilities such as – when / how / what dose was used? eg. if it rained within two hours of pour-on administration – it may be that the product has washed off.

Can we not just use another product if we have resistance?

There are, unfortunately, no new anthelmintic products being brought to market. Ultimately, for now, what we have is all that is available. Once the worms on your farms have built up resistance to all these products, it is likely that parasites may once again become a severe health risk if we are unable to effectively treat livestock. Reducing the risk of resistance on your farm, should be a priority.